if (/^[[:allnum:]]{1,13}$/){
              =~ s/^([[:allnum:]]{1,13})$/$1.'X'/
}

##</code><code>##

if (/^[[:allnum:]]{1,13}$/){
           =~ s/^([[:allnum:]]{1})([[:allnum:]]{1,12})$/$s1 .'X'. $2/
}